1. Field of the Invention
This invention relates to a cold rolled steel sheet suitable for use in, for example, automobile panels and the like requiring an excellent press formability. More particularly, the invention relates to an improvement in the properties of the above steel sheet through combined addition of Nb and B.
2. Description of the Prior Art
The cold rolled steel sheets for use in the above applications are required to have the following material characteristics:
(1) Deep Drawability: The deep drawability is evaluated by a Lankford value (r-value). The r-value of not less than 2.0 is required in case of deeper drawing.
(2) High ductility: A low yield strength (YS) and a high elongation (El) are required in order to achieve this characteristic.
(3) Non-aging property at room temperature: This means that the material is not deteriorated by the age hardening even when it is stored at room temperature for a long period of time.
(4) Resistance to denting: This means that the steel sheet after the press forming does not dent under a light load and is required to have a high yield strength of the steel sheet after the press forming.
Since the value YS is required to be low in the press forming, it is generally difficult to simultaneously realize both the press formability and the resistance to denting. However, it is possible to satisfy such conflicting properties in case of steel sheets having a property that it is hardened by the heating treatment (for instance, baked-on finish) subsequent to the press forming (hereinafter referred to as BH property).
The conventionally known cold rolled steel sheets for press forming are classified as follows:
(1) Steel sheets obtained by box annealing of low carbon aluminum-killed steel: This steel sheet is excellent in the deep drawability, ductility, and non-aging property at room temperature, but has almost no baking hardenability and also the resulting press formed parts are poor in the resistance to denting. Further, since the low carbon aluminum-killed steel is used as a raw material, it is difficult to secure the above-enumerated properties thereof by the continuous annealing method which is considered to be advantageous from the standpoints of the productivity and the homogeneity of the product.
(2) Steel sheets obtained by adding Nb or Ti to an extremely low carbon steel: This steel sheet exhibits excellent deep drawability and ductility even by the continuous annealing as in the case with the box annealing, and has the non-aging property at room temperature. Particularly, it has an extremely deep drawability because the r-value is not less than 1.8. However, it is not easy to provide the BH property likewise the case (1), so that the press formed part is poor in the resistance to denting.
(3) Dual-phase structure steel sheets in which ferrite and martensite phase are made coexistent by adding alloying elements such as Si, Mn, Cr, etc. to low carbon aluminum-killed steel and controlling the cooling rate after the continuous annealing: This steel sheet has the merit that because it has a lower yield strength as compared with the conventional steel sheet, it is excellent in the bulging property and is easy to gain a high strength. Further, it has a non-aging property at room temperature and a high BH property. However, it is poor in the drawability because the r-value is as low as about 1.0.
Although the methods of manufacturing cold rolled steel sheets having a dual-phase structure have hitherto been disclosed in U.S. Pat. Nos. 4,050,959, and 4,062,700, Japanese Patent Application Publication No. 53-39,368, Japanese Patent laid open Nos. 50-75,113 and 51-39,524 and so on, all of them do not relate to a method of manufacturing steel sheets with a high r-value, and are far behind the goal aiming at the invention.